During tune-ups on oil furnaces, we often find the fuel filter coated with a black sludge.  Homeowners often ask where this sludge comes from and if the oil in their tank is “dirty”.  In most cases the sludge is a combination of dirt, tank rust scale and oxidation of the hydrocarbons in the oil that settle to the bottom of the tank.  The problem of sludge can be aggravated when water is present in the tank, water that is often a result of the natural warming and cooling of the tank with changes in temperature allowing condensation to occur.

It is at this water / fuel interface that microorganisms grow, oxidizing the oil and the result is their accumulation as waste in the bottom of the tank with other dirt and rust forming sludge.  In most instances, the pick up line for an oil furnace is placed several inches off the bottom of the tank to reduce the possibility of this sludge being pulled into the supply line.  But, this sludge can find its way to your filter when the sludge is stirred up during routine tank filling or when the sludge reaches the level of the oil supply line.

In our next installment, we will discuss what options are available to either remove or reduce the sludge in oil tanks